The call is_ratconst t tests whether the term t is a canonical rational
literal of type :real. This means an integer literal &n for numeral n,
-- &n for a nonzero numeral n, or a ratio &p / &q or -- &p / &q where
p is nonzero, q > 1 and p and q share no common factor. If so,
is_ratconst returns true, and otherwise false.